I think the answer to the op is 2 fold
first, If you are going get a MIM and customize it, forget it just start from scratch you will end up with a better guitar
Second, is a AS really that much better. I think so, it has better tuners. better hardware, better pickups etc
is that worth 500 more. Depends on when you think to value it. I can buy an MIM any where on Craig's list for 300 dollars used, the same thing in a AS but it cost between 500 to 5000 more. depending on it;s age, the older the more it cost, I know my AS will go up in price, mine sells now for more than I paid for it, the MIM made in the same year goes for 300 dollars used.
So is a AS worth the money, you betcha, is the Mexican strat made with the same hardware and electronics of the American strat. No but is it worth the upgrade money? Hard to tell in today's market, but it also comes with a hardshell case which probably makes it worth the money.